This invention relates to roll fuser apparatus for use in a xerographic reproducing apparatus and particularly, to means for stripping copy paper from the heated roll thereof in order to prevent the paper from curling and wrapping around the fuser roll.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual image can be either fixed directly upon the photosensitive member or transferred from the member to a sheet of plain paper with subsequent affixing of the image thereto.
In order to permanently affix or fuse an electroscopic toner material onto a support member by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This action causes the toner to be absorbed to some extent into the fibers of the support member which in many instances constitutes plain papers. Thereafter, as the toner material is cooled, solidification of the toner material occurs causing the toner material to be firmly bonded to the support member.
In both the electrographic as well as the xerographic recording arts, the use of thermal energy for fixing toner images onto a support member is old and well-known.
One approach to thermal fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is either externally or internally heated.
During operation of a fusing system of the above-described type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll to thereby provide heating of the toner image within the nip. By controlling the heat transferred to the toner and by the provision of proper roll surface materials virtually no offsetting of the toner particles from the copy sheet to the fuser roll is experienced under normal conditions. This is because the heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the hot offset temperature of the toner whereat the toner particles in the image areas of the toner would liquify and cause a shearing action in the molten toner to thereby result in offset. Shearing occurs when the interparticle forces holding the viscous toner mass together is less than the surface energy forces tending to offset it to a contacting surface such as the fuser roll.
Occasionally, however, extraneous toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof, by imperfections in the properties of the entire surface of the roll, or by the toner particles insufficiently adhering to the copy sheet by the electrostatic forces which normally hold them there. In such a case, extraneous toner particles may be transferred to the surface of the fuser roll beyond the nip, with subsequent transfer to the backup roll during periods of time when no copy paper is in the nip and before the backup roll can be moved out of contact with the fuser roll.
It will be appreciated that in order to prevent such toner particles being transferred to the copy paper it is necessary to remove the toner particles from the fuser roll and/or the backup roll. It will be further appreciated that if enough toner accumulates on the backup roll the paper feed will be affected.
One arrangement for minimizing the foregoing phenomena, commonly referred to as "offsetting" has been to provide a fuser roll with an outer covering or sleeve of polytetrafluoroethylene, commonly known as Teflon, to which a release agent such as silicone oil is applied. Silicone based oils, which possess a relatively low surface energy, have been found to be a material that is suitable for use in the heated roll fuser environment. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to thereby form an interface between the roll surface and the toner images carried on the support material. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from adhering to the fuser roll surface. Although the low surface energy oils generally act as non-wetting fluids in regard to most support materials it has been found that a mechanical flowing of the release agent from the roll onto the support material will occur if an excess of oils is allowed to accumulate in a region where it can come into contact with the copy paper. Accordingly, the amount of oil applied to the roll surface is generally metered under controlled conditions to maintain a relatively thin coating of the release agent on the roll surface.
As the copy sheet passes out of the nip formed between the heated fuser roll and the backup roll it is necessary to provide means for insuring that the copy paper proceeds along a predetermined path from the aforementioned nip to a conveyor belt or the like for moving the copy paper toward the exit of the machine.
It has been the practice in prior art devices to employ stripper fingers for the purpose of stripping the copy paper from the heated fuser roll and for also guiding the copy paper from the nip area to the conveyor belt or the like. In order to strip the copy paper by means of stripper fingers it is necessary that the leading edges of the stripper fingers contact the surface of the fuser roll such that they can become interposed between the roll surface and the copy paper. It will be appreciated that such contacting of the fuser roll surface has attendant disadvantages, for example, they cause the fuser roll surface to wear due to frictional contact and sometimes due to gouging of the roll surface, particularly, when the roll is used in a dry (i.e. without release agent) fuser system and where the fuser roll outer surface comprises a high coefficient of friction material such as silicone rubber.
An integral stripping mechanism provided in the surface of the fuser roll member is disclosed in U.S. Pat. No. 3,904,354. In the aforementioned patent the surface of the fuser roll member is provided with a plurality of annular ridges which are deformable as the ridges move through the nip formed between the two fuser roll members to thereby provide a uniform surface in the nip. As the portions of the ridges move out of the nip they resume their normal protruding relationship and thereby effect stripping of the copy paper from the fuser roll member.
Another method of stripping copy substrates from a resilient fuser roll is to apply a brake to the driven roll thereby increasing the torque transmitted between the two rolls which has the effect of stripping substrates from the surface of the roll due to the stresses set up in the roll by virtue of the braking action.
It is the principal object of this invention to provide an improved roll fusing device for fixing toner images to a substrate in an electrostatic reproducing apparatus.
Another object of this invention is to provide an improved means for removing copy paper from the surface of the heated fuser roll.
Yet another object of this invention is to provide means for stripping copy paper from a heated fuser roll wherein such means forms an integral part of the heated fuser roll.